Electrical and Electronic Engineering - Theses
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ItemTransthoracic resistance during cardiac defibrillation( 1983)Before entering into a description of the scope of this project, it is worth mentioning that human transthoracic resistance, not impedance, is the subject of this thesis. The relationship between voltage and current in the human body is known to-vary with both current and time. Complex impedance is an abstraction, and is not useful for describing such a non-linear, time varying system. The concept is best suited to linear, time invariant systems or, at worst, non-linear time invariant systems because of the system model which is implied with complex impedance. Certainly, it may be reasonable to represent the electrical transfer function of the thorax as a combination of non linear phase lead, phase lag and transconductance parameters, but such a model loses significance when the only waveform available for testing is a very slightly underdamped sinusoid (the usual DC defibrillating waveform). Of course, measurements can be taken at low current levels, below a few tens of milliamps, with different source waveforms and with no adverse effects on the subject; however, this does not necessarily give information about what happens at the higher current levels. Voltage and current are the only two "real" parameters available for measurement. The transfer function between these two variables for the human thorax can be completely described for a particular current (or voltage) waveform if they are both measured at each point in time for the duration of the waveform. The term "resistance" will be used hereafter to refer to the instantaneous ratio of this voltage and current. Closed chest defibrillation of the heart is carried out frequently in hospitals as treatment for cardiac arrest, ventricular fibrillation and other cardiac arrythmias. The success rate is not 100%, and since the turn of the century much work has been done to isolate and quantify thevariables affecting the oùtcome of such resuscitation attempts. Most workers now agree that the peak current density in the myocardium during defibrillation is one of the most important factors determining success. It is clear that this will be critically affected by the electrical resistance of the thorax, where the defibrillating electrodes are applied. (From Ch. 1)